1. Field of the Invention
The present invention relates to a superconducting device, and more specifically to a superconducting device having an extremely thin superconducting channel formed of oxide superconductor material.
2. Description of Related Art
Devices which utilize superconducting phenomena operate rapidly with low power consumption so that they have higher performance than conventional semiconductor devices. Particularly, by using an oxide superconductor material which has been recently advanced in study, it is possible to produce a superconducting device which operates at relatively high temperature.
A Josephson device is one of well-known superconducting devices. However, since the Josephson device is a two-terminal device, a logic gate which utilizes Josephson devices becomes complicated configuration. Therefore, three-terminal superconducting devices are more practical.
Typical three-terminal superconducting devices include two types of super-FET (field effect transistor). The first type of the super-FET includes a semiconductor channel, and a superconductor source electrode and a superconductor drain electrode which are formed closely to each other on both side of the semiconductor channel. A portion of the semiconductor layer between the superconductor source electrode and the superconductor drain electrode has a greatly recessed or undercut rear surface so as to have a reduced thickness. In addition, a gate electrode is formed through a gate insulating layer on the portion of the recessed or undercut rear surface of the semiconductor layer between the superconductor source electrode and the superconductor drain electrode.
A superconducting current flows through the semiconductor layer (channel) between the superconductor source electrode and the superconductor drain electrode due to the superconducting proximity effect, and is controlled by an applied gate voltage. This type of the super-FET operates at a higher speed with a low power consumption.
The second type of the super-FET includes a channel of a superconductor formed between a source electrode and a drain electrode, so that a current flowing through the superconducting channel is controlled by a voltage applied to a gate formed above the superconducting channel.
Both of the super-FETs mentioned above are voltage controlled devices which are capable of isolating output signal from input one and of having a well defined gain.
However, since the first type of the super-FET utilizes the superconducting proximity effect, the superconductor source electrode and the superconductor drain electrode have to be positioned within a distance of a few times the coherence length of the superconductor materials of the superconductor source electrode and the superconductor drain electrode. In particular, since an oxide superconductor has a short coherence length, a distance between the superconductor source electrode and the superconductor drain electrode has to be made less than about a few ten nanometers, if the superconductor source electrode and the superconductor drain electrode are formed of the oxide superconductor material. However, it is very difficult to conduct a fine processing such as a fine pattern etching, so as to satisfy the very short separation distance mentioned above.
On the other hand, the super-FET having the superconducting channel has a large current capability, and the fine processing which is required to product the first type of the super-FET is not needed to product this type of super-FET.
It is estimated that superconducting carriers (Cooper pairs) are forced in the direction away from the gate electrode, so that the space charge of a depletion layer in which there is no superconducting particle satisfy the neutrality condition with the charge between the gate electrode and the gate insulator, when a signal voltage is applied to the gate electrode of the super-FET having a superconducting channel. Since the carrier density of an oxide superconductor ranges 10.sup.20 -10.sup.21 /cm.sup.3, which is a medium value between that of a semiconductor and a metal, the depletion layer has a thickness of a few nanometers in case of a superconducting channel of an oxide superconductor and applied voltage of a few volts.
Therefore, in order to obtain a complete ON/OFF operation, the superconducting channel of an oxide superconductor should have an extremely thin thickness of a few nanometers. In addition, it is necessary to process the gate with high precision of sub-micrometers, which has a suitable length relative to its thickness.
However, in the super-FET, special attention is not given to the source-drain distance, namely the sum of the distance between the superconducting source region and the gate electrode and the distance between the gate electrode and the superconducting drain region. Since it has been considered that the critical current density of the superconducting channel is locally changed at a gate portion just under the gate electrode so that the gate portion becomes resistive, when a voltage is applied to the gate electrode. Other portions of the superconducting channel is considered to be superconducting regardless of the voltage which is applied to the gate electrode. Therefore, it is considered that they have no effect on the modulation of the current flowing through the superconducting channel, even if they are very long relative to the gate portion.
However, an oxide superconductor is a superconductor of the second kind so that flux penetrates into it under some conditions, while the oxide superconductor is in the superconducting state. A portion of the oxide superconductor at the center of the penetrating flux is in a normal conducting state and it is resistive. In addition, if large superconducting current flows through the superconducting channel of an oxide superconductor, since the flux migrates in the superconducting channel, a voltage is generated between the both ends of the superconducting channel.
In a super-FET of a prior art, as mentioned above, no attention is given to the distance between the superconducting source region and the gate electrode and the distance between the gate electrode and the superconducting drain region. Therefore, when the superconducting channel is resistive, a voltage applied to the gate electrode can change resistance of only the gate portion just under the gate electrode.
If the superconducting channel is very long relative to the gate portion, the change of the resistance of the gate portion becomes very small relative to the whole resistance of the superconducting channel. Therefore, in this case, even if large modulation is performed at the gate portion, little current modulation and little voltage modulation are observed between the both ends of the superconducting channel.
By this, the conventional super-FET does not have an enough performance. Summary of the Invention
Accordingly, it is an object of the present invention to provide an FET type superconducting device having a superconducting region constituted of an extremely thin oxide superconductor film, which have overcome the above mentioned defects of the conventional ones.
The above and other objects of the present invention are achieved in accordance with the present invention by a superconducting device comprising a thin superconducting channel formed of an oxide superconductor, a superconducting source region and a superconducting drain region formed of an oxide superconductor at the both ends of the superconducting channel which connects the superconducting source region and the superconducting drain region, so that superconducting current can flow through the superconducting channel between the superconducting source region and the superconducting drain region, and a gate electrode through a gate insulator on the superconducting channel for controlling the superconducting current flowing through the superconducting channel, in which the length of the gate electrode ranges from one third of the length of the superconducting channel to one and a half length of the superconducting channel.
In the superconducting device in accordance with the present invention, if the gate electrode has an equal or longer length than that of the superconducting channel, the resistance of the superconducting channel is changed along its whole length when a voltage is applied to the gate electrode. By this, large modulation of both current and voltage of the superconducting channel and complete ON/OFF operation of the superconducting channel are realized.
In case that the gate electrode is shorter than the superconducting channel, only the resistance of the gate portion (the portion just under the gate electrode) of the superconducting channel is changed when a voltage is applied to the gate electrode. However, gate electrode of the super-FET in accordance with the present invention has a length not shorter than one third of that of the superconducting channel. Therefore, the change of resistance of the gate portion is very large compared to the whole resistance of the superconducting channel. Thus, large modulation of both current and voltage of the superconducting channel and complete ON/OFF operation of the superconducting channel are realized as the above-mentioned super-FET.
In the super-FET in accordance with the present invention, the superconducting channel, the superconducting source region and the superconducting drain region can be arranged arbitrarily, so long as the superconducting source region and the superconducting drain region is arranged separately from each other and the superconducting channel connects them.
In one preferred embodiment, the superconducting channel connects the top portions of the superconducting source region and the superconducting drain region and the superconducting channel, the superconducting source region and the superconducting drain region have a planar upper surface. This planar upper surface is desirable to form a gate electrode which is longer than the superconducting channel.
It is also preferable that the superconducting channel connects the middle portions of the superconducting source region and the superconducting drain region and the gate electrode and the gate insulator are bent along the upper surface of the superconducting channel, the superconducting source region and the superconducting drain region. In this type of the super-FET, superconducting current flows into or flows from the superconducting channel efficiently so that the current capability of the super-FET can be improved.
In another preferred embodiment, the superconducting channel connects the bottom portions of the superconducting source region and the superconducting drain region and the gate electrode and the gate insulator are arranged on the superconducting channel between the superconducting source region and the superconducting drain region. The super-FET preferably includes an insulating layer which surrounds side surfaces of the gate electrode and which isolates the gate electrode from the superconducting source region and the superconducting drain region. In this case, the super-FET has a substantially planar upper surface. This planar upper surface is favorable for forming conductor wirings in a later process.
In the super-FET in accordance with the present invention, the superconducting channel preferably connects the top portions of the superconducting source region and the superconducting drain region and the gate electrode is arranged under the superconducting channel through the gate insulating layer between the superconducting source region and the superconducting drain region. The super-FET also has a planar upper surface which is favorable for forming conductor wirings in a later process.
In still another preferred embodiment, the superconducting source region and the superconducting drain region are placed in staggered fashion, and the superconducting channel connects the superconducting source region and the superconducting drain region so that the superconducting source region, the superconducting channel and the superconducting drain region are arranged to form a crank in the named order. In this type of the super-FET, the gate insulator is successively formed on the upper surface of the superconducting source region, on the side surface of the superconducting channel and on the upper surface of the superconducting drain region, and the gate electrode is formed on an adjoining side of the superconducting channel through the gate insulator.
In a preferred embodiment, the oxide superconductor is formed of high T.sub.c (high critical temperature) oxide superconductor, particularly, formed of a high-T.sub.c copper-oxide type compound oxide superconductor for example a Y-Ba-Cu-O compound oxide superconductor material, a Bi-Sr-Ca-Cu-O compound oxide superconductor material, and a Tl-Ba-Ca-Cu-O compound oxide superconductor material.
In addition, the substrate can be formed of an insulating substrate, preferably an oxide single crystalline substrate such as MgO, SrTiO.sub.3, CdNdA10.sub.4, etc. These substrate materials are very effective in forming or growing a crystalline film having a high degree of crystalline orientation. However, the superconducting device can be formed on a semiconductor substrate if an appropriate buffer layer is deposited thereon. For example, the buffer layer on the semiconductor substrate can be formed of a double-layer coating formed of a MgAl.sub.2 O.sub.4 layer and a BaTiO.sub.3 layer if silicon is used as a substrate.
Preferably, the superconducting channel is formed of a c-axis oriented oxide superconductor thin film and the superconducting source region and the superconducting drain region are formed of a-axis oriented oxide superconductor thin films.
The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.